Ultrasonic gauging employs sound waves or mechanical vibrations whose frequency is above the audible range in the frequency spectrum. The sound is produced by a transducer exhibiting piezo electric properties where electrical energy is converted into mechanical vibrations or, conversely, mechanical vibrations are converted into an electrical signal. Thus, the transducer can be used to transmit ultrasonic waves and to detect or receive the waves. The sound produced by the transducer is introduced into the body to be gauged through a liquid couplant such as water or oil, and generally propagates in a fairly well defined beam through the material. The propagation continues until some or all of the sound is reflected by a boundary, for example, the inner wall of a tube.
Specialized techniques have been developed to extend the usefulness of the ultrasonic equipment. One apparatus, known as a bubbler, consists of a semicontained liquid column. The transducer is sealably mounted in one end of the column, and the body being gauged substantially covers the other end. There can be some leakage of couplant where the body covers the column end, and the leakage can be minimized by proper design. The bubbler provides a solid column of water substantially free of air bubbles between the transducer and the body being gauged. A completely free flowing liquid column can be used where the motion or temperature of the body being gauged is excessive.
The pulse-echo technique of ultrasonic gauging employs a short burst of ultrasonic energy known as the initial pulse. It is transmitted into the body by the transducer through the coupling medium. The ultrasonic impulse travels in essentially a straight line until it strikes a reflecting surface such as the oppositely facing tube surface. The ultrasonic reflection of the wave from the surface is governed by well known laws, analogous to the laws of optics. Any of the reflected energy that returns to the transducer is detected as an echo signal, and its amplitude and location in time are related to the thickness of the tubing wall. To measure the wall thickness of a cylindrical body, the pulse-echo ultrasound measurement requires precise alignment and focusing of the ultrasound beam with the body.
For example, ultrasound can be used to measure the wall thickness of a tube by timing the interval between reflections of the sound wave from the inner and outer surfaces of the wall. The transducer must be aimed so that the beam of ultrasonic waves is directed radially to the cylinder axis, herein referred to as the reference axis, and focused at a preselected location within the wall or inner diameter of the tube. Misalignment of the transducer can cause attenuation of the reflected sound waves, and an incorrect measurement of the wall thickness.
One aspect of this invention is to provide an apparatus for aiming a transducer in an ultrasonic gauging apparatus.